Causal associations and predictability of the summer East Atlantic teleconnection

Abstract. We apply Causal Effect Networks to evaluate the influence of spring North Atlantic extratropical surface temperatures (SST) on the summer East Atlantic Pattern (EA) seasonal predictability during the 20th century. We find in the ERA-20C reanalysis that a meridional SST gradient in spring (SST index) causally influences the summer EA, with an estimated causal effect expressed by a β-coefficient of about 0.2 (a 1 standard deviation change in spring SST index causes a 0.2 standard deviation change in the EA 3–4 months later). We only find this link to be causal, however, during the period 1958–2008. When performing the analysis on 45-year-long timeseries randomly sampled in this late period, we find the strength of the causal link to be affected by interannual variability, suggesting a potential modulation by an external physical mechanism. In addition to the summer EA, we find that spring SST has an estimated causal effect of about -0.2 on summer 2-metre air temperatures over northwestern Europe, possibly mediated by summer EA. We then use a pre-industrial and a historical simulation, as well as a 30-member initialised seasonal prediction ensemble with MPI-ESM-MR to test the model performance in reproducing the detected causal links in ERA-20C and to evaluate whether this performance might leave an imprint in the model predictive skill of European summer climate. We find that while MPI-ESM-MR is mostly unable to reproduce the causal link between spring SST and the summer EA among the different datasets, the 30-member initialised ensemble can moderately reproduce a causal link between spring SST and summer 2-metre air temperatures over a region west of the British Isles. We perform a predictive skill assessment conditioned on the spring SST causal links for July–August sea level pressure, 500 hPa geopotential height and 2-metre air temperatures for predictions initialised in May. Our results suggest that MPI-ESM-MR's performance in reproducing the spring SST causal links constrains the seasonal prediction skill of European summer climate.